Almost a third of kids diagnosed with attention-deficit hyperactivity disorder during childhood still had it as adults, and more than half also had another psychiatric disorder, finds one of the largest studies to follow children with ADHD into adulthood.

Twenty-nine percent of children diagnosed with ADHD still had the condition at age 27, says the study in April's Pediatrics, released online Monday. Nearly 57% of those diagnosed during childhood had at least one other psychiatric disorder as adults, compared with 35% of those in a comparison group who did not have childhood ADHD.

"The risk for persistent ADHD is considerable, and the risk for at least one mental health condition, including ADHD, is extraordinarily high," says lead investigator William Barbaresi, a developmental medicine specialist at Boston Children's Hospital. "Only 37.5% of the children we contacted as adults were free of these really worrisome outcomes."

The study also found the risk for suicide was nearly five times higher among those diagnosed with ADHD than in the comparison group, and nearly 3% of study participants were in jail when recruited for the adult portion of the study.

ADHD is one of the most common neuro-developmental disorders of childhood, affecting as many as 9% of kids ages 3 to 17 (5 million children) and between 2% and 4% of adults, according to the National Resource Center on ADHD. Symptoms include excessive inattention, hyperactivity and impulsivity.

Other studies have also highlighted long-term effects of ADHD, but this one "is particularly telling because it used a community sample of children with ADHD followed to adulthood and not a clinical sample of individuals seeking treatment for their problems," says J. Russell Ramsay, co-director of the University of Pennsylvania's Adult ADHD Treatment and Research Program in Philadelphia. He was not involved in the study.

"It is chilling to see evidence, at least in this study, of the increased risk for death by suicide among children diagnosed with ADHD, with most of these tragic cases also having a history of substance abuse and at least one co-existing psychiatric diagnosis," Ramsay says.

Conducted with researchers at the Mayo Clinic in Rochester, Minn., the study drew from all 5,718 children born between 1976 and 1982 and still residing in Rochester at age 5 and whose families allowed access to their medical records. Among that population, 367 who had documented ADHD participated; 232 who met the study's criteria for adult ADHD participated in the follow-up study at age 27.

Although 29% of the childhood ADHD cases continued to have ADHD as adults, 71% did not, and it's unclear what accounts for that difference, Barbaresi says.

He notes that among children whose ADHD persisted into adulthood, 81% had at least one additional mental health problem. Among those whose childhood ADHD did not persist, 47% had at least one other mental health problem. One preliminary suggestion, he says, is that having "an associated mental health problem" may increase the likelihood that ADHD will persist into adulthood.

The cause of ADHD is unknown, but current research shows that genetics play an important role, according to the Centers for Disease Control and Prevention. Symptoms can be controlled by a combination of behavioral therapy and medication.

This study "shows that the adverse effects of ADHD are persistent and long-term in a substantial proportion of those with a childhood diagnosis," says Tanya Froehlich, a developmental-behavioral pediatrician at Cincinnati Children's Hospital Medical Center. She was not involved in the study. The disorder and its associated conditions "have serious public health consequences," she says.

When it comes to treatment, ADHD warrants the same approach as chronic health problems such as diabetes, Barbaresi says. In those cases, "when a child is diagnosed, we immediately institute strategies aimed at keeping the child engaged in appropriate treatment for the long haul."

For many reasons, including "a huge problem with regard to health care coverage of appropriate assessment and treatment ... that's not really happening at a systematic level with ADHD," he says.

Some insurance plans categorize ADHD as a behavioral condition and not a medical condition, thereby limiting coverage for treatment, says Tim MacGeorge, director of the National Resource Center on ADHD.